The larva of a hexactinellid sponge. (Illustration by Emily Damstra)
laminar proteinaceous structure. While it undoubtedly serves as a partial barrier to the free exchange of materials, electric currents can flow through it and transport vesicles are able to move through pores in its structure. Between cellular and syn-cytial components of the sponge there is a very thin collagen layer, the mesohyl. This layer is believed to be too thin for cells to migrate within, as is the case with other sponges. Instead, transport of nutrients and other materials appears to occur along vast networks of microtubules within the multinucleate tissue.
Hexactinellid sponges are known at depths from 30 to 22,200 ft (9.14-6,770 m) in all oceans. There are no records of this class in freshwater. The fossil record suggests that their historical range was similar.
The vast majority of hexactinellids live at depths greater than 1,000 ft (304.8 m). In a few coastal locations, however, such as Antarctica, the northeastern Pacific, New Zealand, and some caves in the Mediterranean, species are found at depths accessible by scuba divers. These habitats have in common cold water (35-52°F, or 2-11°C), relatively high levels of dissolved silica, and low light intensity. Although many hexactinellids require a firm substratum, such as rocks, for attachment, others grow on fused skeletons of dead sponges, and still others live over soft sediments. The latter group, though not numerous, support themselves on struts made of bundles of long spicules that project down into the sediment.
Sponges are not noted for their complex behavior. Nevertheless, hexactinellids can respond to mechanical or electrical stimuli by instantly shutting down the feeding current. The explanation for this unusual ability lies in their possession of a trabecular reticulum, which acts like a nervous system, conveying impulses to all parts of the body. Sponges of other groups lack any such system and show no evidence of an ability to conduct electrical impulses. Electrical signals traveling at 0.07 in per second (0.26 cm per second) have been recorded from slabs of the body wall of R. dawsoni. It is presumed that when the signals reach the flagellated chambers, the pumping stops. No rhythmic pattern has been found in the cessation of pumping. It is thought that since glass sponges lack motile cells that would otherwise remove unwanted material from the sponge, shutting down the feeding current may prevent the clogging of the canal system with large amounts of debris.
Like the majority of sponges, glass sponges are thought to filter food from the water that they pump through the choanosome. Two in situ studies confirm this to be true for sponges that lack debris on their outer surfaces. Studies comparing the content of inhaled and exhaled water showed that both Aphrocallistes vastus and Sericolophus hawaiicus retain particulate matter—mostly bacteria—and rely little on dissolved organic carbon. The results of one study that compared such water samples from a sponge covered in debris (Rhabdocalyp-tus dawsoni) suggest that particulate matter is not retained and that the sponge instead relies for nutrition entirely on the uptake of dissolved organic carbon. It is thought that the organisms coating the sponge produce sufficient organic carbon for
Glass Sponge flagellated chamber dermal membrane
themselves and their host. Nevertheless, R. dawsoni can phagocytose both bacteria and latex beads in laboratory preparations.
Laboratory experiments with R. dawsoni and Oopsacas minuta (also a rosellid sponge, but one with a clean exterior) have shown that uptake of particulates occurs in the trabecular syn-cytium near to and in the flagellated chambers, not in choanocytes, as is normally the case in cellular sponges. In hexactinellid sponges the structure equivalent to a choanocyte, the collar body, lacks a nucleus, and in most species examined so far the collar body is enveloped by extensions of the tra-becular syncytium (the primary and secondary reticula), which do most of the particulate capture and uptake. The siliceous skeleton may protect hexactinellids from many predators, but at least one asteroid species is not deterred. Pteraster tesselatus frequently can be found digesting the soft tissues off the skeleton of R. dawsoni.
It is generally thought that most hexactinellids lack a seasonal reproductive period because of their deepwater habitat. Nevertheless, because of the difficulty in collecting and preserving these sponges there is little information on reproduction in most deepwater populations. Our knowledge of their development comes from studies done on only a handful of species. Hexactinellid sponges are viviparous. Eggs arise from cells within groups of archaeocytes, a type of pleuripotent cell found in all sponges. The first cell divisions that occur are equal and result in the formation of a hollow ball of cells (a blastula). Gastrulation (the formation of two cells layers during embryonic development) is said to occur by delamination. The larvae are top-shaped with a girdle, band, or cilia around their middle. The tissues are already syncytial, although it is not yet known how the multinucleate tissue arises. When the larva matures, it is released through the osculum. The species Oopsacas minuta, which is reproductive throughout the year, produces the only known live larvae. When studied in a laboratory setting larvae swim slowly to the surface of dishes in left-handed rotations (clockwise, as seen from the anterior pole); although they can swim for several days, they begin to settle and transform into juvenile sponges within 12 hours of release from the parent sponge.
In general, most hexactinellid sponges inhabit areas well out of the reach of human activity. However, on the continental shelf of the northeastern Pacific in British Columbia, reefs of hexactinellid sponges several city blocks in area have been damaged by trawlers. New legislation for the establishment of marine protected areas around these reefs is under development. No species of hexactinellid is listed by the IUCN.
Euplectella aspergillum, which harbors a pair of crustaceans within its enclosed atrial cavity for life, is commonly given to newlyweds in Japan as a symbol of bonding.
1. Cloud sponge (Aphrocallistes vastus); 2. Neoaulocystis grayi; 3. Glass-rope sponge (Hyalonema sieboldi); 4. Farrea occa; 5. Venus's flower basket (Euplectella aspergillum); 6. Monorhaphis chuni; 7. Bird's nest sponge (Pheronema carpenter!); 8. Sharp-lipped boot sponge (Rhabdoca-lyptus dawsoni). (Illustration by Emily Damstra)
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